In recent telephony systems, it has become common to transmit information to a called telephone while the called telephone is in the on hook state; i.e.; prior to the telephone being answered. A common example of this technique is what is known as caller identification or caller ID. In a caller ID system, the telephone number of the calling party is transmitted to the called telephone and is displayed prior to the called telephone being answered.
There are several common techniques of transmitting information to the called terminal prior to the called terminal being taken off hook. One signaling scheme involves the use of a varying polarity in a voltage presented across the tip and ring terminals of the called telephone in order to transmit digital data. For example, the polarity of the voltage, or the change in the polarity of the voltage, may represent ones and zeroes and may be interpreted to convey digital data. Such systems are known and in use today.
One problem with such systems is caused by the fact that the end user of the called telephone equipment must be isolated electrically from the telephone network itself. This is required in order to ensure that power surges, such as those caused by lightning, are not transferred through to the end user equipment and possibly to a human in contact with such equipment.
In order to provide such isolation, typically optocouplers are used. An optocoupler comprises a first device that turns an electrical current into a light signal, and a second device that converts the light signal back into an electrical current. If a strong power surge occurs on the telephone line, the isolation keeps the high voltage from reaching the end user.
The optocoupler requires a current through a diode in order to emit light. In order to ensure that all changes in the tip and ring voltage are accurately captured, the system must be arranged to present a relatively low impedance to the telephone system. If the impedance presented by the end user telephone equipment is too high, then the current will not be significant enough to ensure accurate capture of the data being conveyed through the optocoupler. On the other hand, if the impedance that is presented to the telephone system is too low, the other end of the telephone line, such as a central office will not be able to distinguish between the on and off hook state.
In view of the widespread use of polarity modulation (i.e; the varying of the polarity of a voltage) to transmit data to an on-hook receiving telephone equipment, there exists a need in the art for an improved technique of accurately detecting the polarity of a voltage presented to an end user telephone equipment. Preferably, the system should present a high impedance while providing high accuracy in the detection of polarity.